FIELD OF THE INVENTION
The present invention relates to a honeycomb body, in particular a catalyst carrier body, for catalytic conversion of exhaust gases in an exhaust system. Such honeycomb bodies are preferentially used in motor vehicles having Diesel or Otto engines. The present invention also relates to an apparatus having the honeycomb body for cleaning exhaust gas.
Honeycomb bodies of the type referred to above are known in a great number of different versions. They may be made and in particularly extruded, from ceramic material. Another type of honeycomb body includes wound, stacked and/or intertwined layers of sheet metal, at least some of which are structured. The flow resistance of a channel in such a honeycomb body depends on its shape and the structure of its walls. Its shape and size determine a hydraulic diameter, while structures and/or obstacles at the ends and/or in the interior of the channel can further increase the flow resistance.
In pipe systems, flows generally have an approximately parabolic velocity profile, at which the flow velocity is highest in the middle and zero at the edge. It is also already known to construct honeycomb bodies in such a way that the flow resistance in a central region is greater than in outer regions. Such honeycomb bodies are described in Published European Patent Application 0 336 106 B1 and Published European Patent Application 0 542 805 B1, corresponding to International Patent Application WO 92/02717, for instance. The prior art generally assumes that the flows are approximately rotationally symmetrical, and therefore honeycomb bodies with different channel cross sections always have an approximately symmetrical, generally concentric configuration of channels of different cross sections. In honeycomb bodies of oval or elliptical cross section, corresponding configurations with two axes of symmetry at right angles to one another are the goal.
It is also known from International Patent Applications WO 90/08249, corresponding to U.S. Pat. No. 5,157,010, WO 91/01178 and WO 91/01807 to provide structures in the interior of channels of a honeycomb body that increase the flow resistance. In the case of the present invention, reference is expressly made to the honeycomb bodies and production processes described in those disclosures.
In Published European Patent Application 0 542 775 B1, corresponding to U.S. Pat. No. 5,328,774, a honeycomb body is also described which has an increasing number of channels in the flow direction per unit of cross-sectional area, even though it is monolithic in structure. Those structural forms are likewise significant to the present invention, and therefore that disclosure is also expressly incorporated by reference. It has been found that in some exhaust systems, symmetrical flow conditions do not prevail, especially in the area near the engine. Due to installation conditions, honeycomb bodies that are used in catalytic converters, especially in the area near the engine, experience an eccentric and/or asymmetrical oncoming flow, so that at certain points, especially heavy loads on the end surface of the honeycomb bodies with consequent damage and shortened service lives are the result.
When electrically heatable honeycomb bodies are used as well, there is an uneven oncoming flow to a downstream honeycomb body, since for purposes of electrical subdivision electrically heatable honeycomb bodies can have air gaps, which have a lesser flow resistance than the channels of the electrically heatable honeycomb body.
An apparatus for catalytic conversion of exhaust gases in an exhaust system, which is known from International Patent Application WO 92/13636, corresponding to U.S. Pat. No. 5,382,774, has at least two spaced-apart honeycomb bodies, disposed one after the other in terms of the flow direction of an exhaust gas. Each honeycomb body has channels through which a fluid can flow. Near the flow axis, support elements are provided, by which the honeycomb bodies are joined together and braced against one another. The honeycomb body which is first in terms of the flow direction is electrically heatable. The electrically heatable honeycomb body is electrically subdivided over its cross-sectional area by ceramic insulating layers, causing an electric current to flow along a path defined by the subdivision. The electrically heatable honeycomb body has the task of minimizing pollutant emissions during a cold-starting phase of an internal combustion engine.
It is also known to provide the electrical insulation of the electrically conductive path through a heatable honeycomb body, relative to adjacent portions, by using an air gap. Such a configuration is described in an article by P. F. Kuper et al entitled "Ultra-Low Power Electrically-Heated Catalyst System", in SAE Technical Papers Series 940465. The air gap brings about an uneven flow through the honeycomb bodies, since some of the fluid to be converted flows through the air gap as a function of the structure of the individual honeycomb bodies. That would occur in particular if the size of the channels in the first honeycomb body were made similar to or smaller than the gap width, which has therefore been avoided until now. Due to that fact, some of the fluid in the cold-starting phase is unconverted or only inadequately converted, or else a portion of the downstream honeycomb body is more heavily loaded.